Endometriosis is an estrogen dependent gynecologic chronic disease defined as the presence of endometrial stromal or glandular cells outside the uterine cavity (31). Endometriotic lesions are most commonly located in ovaries, the pelvic peritoneum and the uterosacral ligaments, but may appear in almost any part of the body (2). Those lesions can vary from a small number of vesicles sticked to the pelvic peritoneum (minimal or mild stage of the disease) to the presence of endometriotic ovarian cysts, thick pelvic adhesions or profound rectovaginal endometriosis (advanced stage of the disease). Endometriosis severity is classified according to the American Society for Reproductive Medicine classification (ASRM) in four stages: I-Minimal, II-Mild, III-Moderate and IV-Severe.
Endometriosis affects up to 10% of women of fertile age, being the major cause of infertility (the incidence in women with infertility rises to up to 40%). Other common symptoms include dysmenorrhoea (pelvic pain with menstruation), dyspareunia (pain with intercourse) and chronic pelvic pain (51).
Despite the identification of endometriosis in the late 1800s, the etiology and pathogenesis of this disease still remains unclear (23). Several theories have been proposed to explain the pathogenesis of this disease, but the most accepted theory is the one proposed by Sampson about the retrograde menstruation. According to this theory, endometrial cells are refluxed through the fallopian tubes during the menstruation and implant onto peritoneum or pelvic organs. But the retrograde menstruation is a very common phenomenon among women of reproductive age (occurs in 90% of women) and only 10% develop endometriosis, so there must be other factors that may contribute to the pathophysiology and/or pathogenesis of endometriosis. Genetic predisposition, environmental factors, and alterations in immune and endocrine functions are believed to play significant roles in the establishment and maintenance of endometriosis.
Although the eutopic endometriums of women with and without endometriosis are histologically similar, studies revealed that there are many differences between these two tissues. Invasive properties, decreased apoptosis, alterations in expression of specific gene and proteins, and increased steroid and cytokine production have been identified in eutopic endometrium of women with endometriosis (69).
Currently, laparoscopy offers the most specific and sensitive technique for evaluating and monitoring endometriosis. Even so, microscopic or occult endometriosis may be misdiagnosed because of the inability to visualize the lesions (77). Furthermore, it has been described that one-third of all diagnostic laparoscopies reveal endometriosis, one-third reveal no visible pathology, and the remaining one-third demonstrate a variety of other gynecologic conditions. Thus, two-thirds of all patients who undergo this invasive diagnostic procedure will not have endometriosis (17).
Actually, the average delay in the diagnosis of this disease is 9.3 years (18). Attempts for early diagnosis and treatments of endometriosis have been weighed down by lack of proper methods to study and manage the disease. Furthermore, the need for non-invasive diagnostic methods is evident because the laparoscopy is a surgical procedure with potentially dangerous risks, such as vascular or intestinal injury (33).
Therefore, the analysis of differential protein expression, using proteomic approaches, in relation to the disease onset and progression will lead the development of non-invasive method for early diagnosis of endometriosis disease.